A principal object of the present invention is to provide practical synthesis for fluorocarbon ether elastomers for use in wide temperature band environments.
The thermal stability and low temperature flexibility of fluorocarbon ether linkages were recognized in the early 1950's. Various studies have shown that the fluorocarbon ether system is flexible at temperatures below 0.degree. C., resistant to organic fluids, and thermally stable in the +300.degree. C. to 400.degree. C. range. The major problems have been, however, low molecular weight and lack of a curing system which gives a cross-linking bridge with stability equal to that of the polyeiher backbone.
Elastomeric polymers have been prepared from difunctional prefluoropolyether prepolymers by cross-linking through condensation or addition reactions. However, all of these systems have had at least one deficiency which has made them unsatisfactory for purposes requiring a wide temperature band and stability under hydrolytic and oxidative conditions.
Polyether elastomer systems which have perfluoropolyether segments or blocks in the backbone utilize perfluoro polyethers derived from perfluorinated epoxides, e.g., hexafluoropropylene oxide and tetrafluoroethylene oxide. Except for polycarbonyl fluoride systems, no other method for producing functionalized perfluoropolyethers is known. All polymerizations of perfluorinated epoxides are of an anionic nature. More specifically, they are all fluoride catalyzed polymerizations even though some other nucleophile may have been used as the initiator. These polymerizations involve high rates of chain transfer (fluoride ion exchange) and consequently relatively low molecular weight materials are obtained.
This is especially evident in the hexafluoropropylene oxide (HFPO) system when the desired products are to be difunctional. After only a few epoxide units have been added to the perfluorodiacid fluoride, monofunctional homooligomers of the epoxide begin to arise. This is intolerable because difunctionality is the highest importance for prepolymers. Maximum difunctionality can be assured in the HFPO system if low molecular weight materials of no more than eight perfluoro-oxyalkylene units are added to the diacid fluoride. Difunctional oligomer materials can be isolated from the homooligomers by careful fractional distillation. Polymers deriving their perfluoropolyether segment from HFPO do not maintain flexibility well at subzero temperatures as low as about -50.degree. C.
Tetrafluoroethylene oxide (TFEO), the only other practical epoxide useful in making perfluoropolyethers is not commercially available because of its instability. Although it can be stored for several months at -78.degree. C., at higher temperatures or even at -78.degree. C. under improper conditions, it will rearrange to trifluoroacetyl fluoride. Under proper condtions, TFEO can be made and polymerized to materials which are very useful for the purposes hereof. See for example U.S. Pat. No. 3,250,806. It has been found that TFEO is especially useful for oligomerization using perfluoro-oxaglutaryl fluoride (Example XLII below). With this acid fluoride, TFEO reacts to give polyfunctional oligomers of very narrow molecular weight distribution and without any monofunctional homooligomers. These results are not both achieved in the oligomerization of TFEO with other acid fluorides. Although in several instances the reaction does not yield homooligomers, it does give oligomers of a fairly wide molecular weight distribution.
Elastomeric polymers which derive their perfluoropolyether segment from TFEO remain flexible at temperatures lower than similar elastomers which derive their perfluoropolyether segment from HFPO, and hence, where lower temperatures are encountered at which flexibility is required, the TFEO derived elastomers are preferred.
Perfluoropolyethers prepared from hexafluoropropylene oxide and tetrafluoroethylene oxide are described in the U.S. Pat. Nos. to Warnell 3,250,806 and Gerhard et al 3,250,807 which are incorporated herein by reference.
It has further been found that quinoxalines provide a suitable backbone member in the polymer because of their stability to heat and oxidative conditions. The alpha-diketones and bis(alpha-diketones) reactive with o-arylene diamines to give the quinoxalines, are conveniently produced starting with the keto-ylids and subsequently chlorinating to yield the dichloroketones and bis(dichloroketones). The quinoxalines hereof are useful to produce by cross-linking, polyquinoxalines which remain flexible over the desired wide temperature band and have the desired stability characteristics.